The present application relates to the field of optical metrology and more particularly to optical metrology that may be performed in the vacuum ultraviolet (VUV).
In one embodiment, a means by which accurate and repeatable optical metrology may be performed in the vacuum ultraviolet (VUV) is provided. In one embodiment, the technique disclosed herein can be used to ensure that vacuum ultraviolet reflectometers generate highly stable and repeatable results in the presence of both gaseous and surface contaminants. In another embodiment, the techniques disclosed herein provide a means for obtaining accurate reflectance data from samples whose surfaces themselves may be contaminated.
Optical metrology techniques have long been employed in process control applications in the semiconductor manufacturing industry due to their non-contact, non-destructive and generally high-throughput nature. The vast majority of these tools operate in some portion of the spectral region spanning the deep ultraviolet to near-infrared wavelengths (DUV-NIR generally 200-1000 nm). The continuous push towards developing smaller devices comprised of thinner layers has challenged the sensitivity of such instrumentation. An effort to develop optical metrology equipment utilizing shorter wavelengths (below 200 nm), where greater sensitivity to subtle changes in processing conditions can be realized has been considered. Approaches to performing optical measurements at shorter wavelengths such as a system and method for a vacuum ultraviolet (VUV) reflectometer are described in U.S. application Ser. No. 10/668,642, filed on Sep. 23, 2003, now U.S. Pat. No. 7,067,818 and U.S. application Ser. No. 10/909,126, filed on Jul. 30, 2004, now U.S. Pat. No. 7,126,131 the disclosures of which are both expressly incorporated herein by reference.
Contamination of optical surfaces like windows and mirrors is a serious impediment to the operation of optical instruments in the VUV. Moisture and residual molecules, particularly hydrocarbon compounds, may deposit on such surfaces over time dramatically reducing their performance. These effects have formed the focus of previous investigations owing to their impact on the design, development and performance of 193 and 157 nm lithographic exposure tools.
In order to ensure the tremendous sensitivity enhancements theoretically offered by VUV optical metrology instruments are practically realized, it would be highly desirable to develop an instrument with the inherent capability of reducing, removing or altogether eliminating the build up of contaminates on its optical surfaces. Furthermore, if this self-cleaning capability could be realized without the addition of potentially expensive and complicated components it would represent a great benefit to tool owners.
When present on the surfaces of samples under investigation, contaminate layers may significantly contribute to measured optical responses in the VUV yielding inaccurate and/or erroneous results. These effects are of particular concern when the samples are comprised of ultra thin films (<100 Å), whose thicknesses may themselves be comparable to the thicknesses of the contaminate layers.
One technique contemplated for improving the measurement of semiconductor wafers by removing contamination layers in a cleaning step includes employing microwave radiation and/or radiant heating, prior to measurement. Although enhanced measurement repeatability is reported using this approach, the method requires that a separate cleaning system be coupled to an existing measurement system resulting in increased system cost and design complexity.
In light of these disadvantages it would be desirable to develop a measurement system that was itself capable of removing contaminants from the surface of samples, so as to ensure accurate and highly repeatable results were achieved. Such an instrument would be capable of simultaneously cleaning and measuring specific locations on the sample without requiring additional components, above and beyond those normally required for measurement, thereby reducing system cost and design complexity. Furthermore, such an instrument would not require alignment of separate cleaning and measurement subsystems. In addition, such an instrument would avoid needlessly “cleaning” the entire sample, while at the same time ensuring that consistent cleaning results were obtained at all measurement locations.